I. Field of the Invention
This invention relates generally to cardiac stimulating apparatus, and more particularly to a method and apparatus for continuous measurement of the impedance presented to the implanted pulse generator apparatus by a pacing and/or defibrillating lead.
II. Discussion of the Prior Art
Although many implantable cardiac rhythm management systems provide data concerning lead function, including pulse voltage, current, charge and energy, the measurement that is used most frequently is that of lead or stimulation resistance (impedance). Changes in lead impedance affect the other measures of lead function.
The terms xe2x80x9cresistancexe2x80x9d and xe2x80x9cimpedancexe2x80x9d, although technically different, are often used interchangeably by the clinical community. Impedance is a complex concept reflecting a changing environment involving a variety of factors. This results in fluctuations in the moment-to-moment resistance. The resistance to electron flow in a pacing system progressively rises during the delivery of the stimulation pulse as a result of polarization at the electrode-tissue interface and, as a continuously changing variable, is appropriately termed impedance. The actual resistance to current flow imparted by the conductor coil is fixed and represents a small portion of the total stimulation resistance. The polarization at the electrode-tissue interface, which is due in part to the surface area and geometry of the electrode, and the impedance associated with conduction of the pulse through the body""s tissues play a larger role in the overall resistance of the system. All this is incorporated in the single measurement termed either lead impedance, or more accurately, stimulation impedance.
Stimulation impedance is affected by many factors, not the least of which are electrode size, configuration and materials. Manufacturers have designed electrodes with high impedance values. For any given output, a high impedance system reduces the overall current drain of the battery and effectively increases the unit""s longevity. Other leads have been designed with low polarization to allow for detection of capture with each pace stimulus. Polarization and impedance are not same phenomenon, although one affects the other. For any given lead model, there is a range of normal impedance values that may be broad, whereas for a specific lead within that model series, the impedance should fall within a relatively narrow range.
The clinician can use knowledge of the lead impedance to follow and identify a developing mechanical problem with the lead. This requires baseline or historical data to recognize subtle changes that may reflect a conductor fracture or a breach of the insulation. It is essential to know what device is being used to make these measurements. As noted previously, different devices may obtain these data at different points of the pacing stimulus. Because of these differences, the impedance measurement obtained with a pacing system analyzer at the time of implantation may be significantly different from that obtained by telemetry from the implanted pacemaker moments, if not years, later. This difference does not necessarily imply a problem. Furthermore, impedance may evolve over time, with a fall in impedance occurring in the days to weeks after implantation, followed by a gradual rise toward the initial measurements on a chronic basis.
Multiple factors may affect impedance, particularly in a unipolar system. For example, measurements obtained during deep inspiration may significantly differ from those obtained during maximal exhalation. In the same patient, impedance measurements obtained that are based on a single output pulse may vary by 100 ohms or even more during the same follow-up evaluation while remaining consistent with normal function. If a marked change in lead impedance from previous measurements (e.g., more than 300 ohms) is encountered during a routine follow-up evaluation, further evaluation of the pacing system is advisable, although even these changes may be normal. If the patient has no clinical symptoms and has stable capture and sensing thresholds, operative intervention would be premature, although a more frequent follow-up schedule might be prudent. A dramatic change in the telemetered lead impedance in the presence of a clinical problem, however, directs the physician toward the likely source of the difficulty.
A dramatic fall in impedance may reflect a break in the insulation, especially in the case of a unipolar lead. This effectively increases the surface area of the electrode, resulting in lower impedance. In a unipolar system, an insulation problem provides an alternative pathway for current flow, starting closer to the pulse generator and resulting in less energy reaching the heart, possibly causing loss of capture. The amplitude of the stimulus artifact, as recorded by an ECG, is determined by the distance the current travels in the tissue from the cathode (tip electrode) to anode (ring electrode or housing of the pulse generator). Hence, a bipolar pacing system in which both active electrodes are inside the heart, separated by only one to two centimeters, results in a small stimulus artifact, whereas the pacing spike recorded in a unipolar system, in which the current travels from the tip electrode to the housing of the pulse generator is large despite equivalent output settings. It is also affected by the recording system: some of the newer digital designs result in a marked signal-to-signal variation in amplitude or in the generation of a uniform amplitude artifact, with any high-frequency electrical transient precluding differentiation of a bipolar and unipolar pacing system based on the analysis of the ECG recording.
In a previously stable cardiac rhythm management system, a mechanical problem developing with the leadxe2x80x94either a breach in the insulation or a conductor fracturexe2x80x94results in a change in the stimulation impedance, which may be reflected by a change in the ECG recorded stimulus artifact. In a bipolar pacing system, an insulation defect between the proximal conductor and the tissue of the body is not likely to affect capture thresholds, but it results in a larger stimulus artifact, making it appear unipolar. Depending on the actual location of the insulation fracture in either the bipolar or unipolar lead, stimulation of the extra cardiac muscle contiguous to the insulation defect may occur. Insulation fractures may also attenuate the electrical signal reaching the pacemaker, possibly resulting in sensing failure.
An increase in lead impedance may be the result of a conductor fracture or a connector problem. When this occurs, the lead impedance often rises to high levels. It is inappropriate, however, to assume that a normal lead impedance is 500 ohms. New leads are being introduced that are designed to be high impedance with values ranging from 1500 to 2500 ohms. Other leads, at implantation, have a relative impedance level in the range of 300 ohms and even 200 ohms. Thus, it is essential to look for a trend in serial lead impedance measurements in conjunction with the stability or changes in capture and sensing thresholds. A mechanical problem with the leadxe2x80x94either a conductor fracture resulting in a high impedance or an insulation failure resulting in a low impedancexe2x80x94eventuates in an overall clinical problem that can be identified by telemetric measurement of the stimulation impedance. When the impedance is sufficiently high, there is no current flow and no effective output, although the telemetered event markers indicate an output and therefore loss of capture. The reduced current flow also results in a fall in the measured current drain of the battery. Any problem, however, may be intermittent. This typically occurs when the two broken ends make contact at times but are separated at other times, or in the case of an insulation failure, when lead movement either opens the compromised area or pushes the edges of the break together resulting in normal function.
Some prior art pacemakers have been able to report lead impedance measurements on a beat-by-beat basis, allowing the physician to observe the digital read-out of lead impedance on a programmer""s screen over a protracted number of cycles. However, such systems have been wasteful of battery current. Here, reference is made to U.S. Pat. No. 5,741,311, which requires application of an AC drive current burst after each pacing pulse.
It can be seen from the foregoing, then, that assessment of lead integrity is essential to patient care and every implant or follow-up evaluation of an implanted device should include a review of such lead integrity by appropriate lead impedance measurement.
Historically, there has been a great deal of overhead associated with making lead impedance measurements. Typically, dedicated sampling networks and algorithms are used to provide a measure of lead impedance by forcing a known signal through the lead-tissue interface and measuring the resultant voltage across the lead terminals. Such methods require significant amounts of analog and digital circuitry and include firmware and software complexities. Moreover, there is an impact to manufacturing and test, since shifts in processed parameters frequently reduce product yield or cause a reassessment of test limits. As an example, reference is made to U.S. Pat. No. 6,044,294.
A need, therefore, exists for a method to measure lead impedance without requiring additional dedicated circuitry to obtain the measurement. The method described herein provides accurate impedance measurement results with a minimum of overhead to the implanted device and programmer. This allows for the addition of other features within the pulse generator for the same given device size. That is, the method of the present invention allows a reduction in circuitry/firmware while permitting accurate impedance measurements to be obtained.
The instant invention provides a new apparatus and method for measuring the impedance of a medical lead used in combination with an implantable pulse generator of the type including a battery-powered switching converter that delivers electrical energy to a pacing capacitor where the pulse generator""s stimulating output pulse is periodically delivered from the pacing capacitor. Logic in the pulse generator is arranged to tally a number of switching cycles of the switching converter that is needed to replenish the energy removed from the pacing capacitor upon delivery of a stimulating pulse to the cardiac tissue. An algorithm is then executed in which lead impedance can be determined as a function of the tally of the number of switching cycles needed to replenish the energy removed from the pacing capacitor upon delivery of the stimulating pulse to the cardiac tissue.